Add support for framework plugins

Darwin platforms are using frameworks to embed libraries. Some of them like iOS and tvOS even forbid the use of dynamic libraries that are not wrapped into a framework.

This merge request adds the support of loading bundles and dylibs from frameworks in a restricted way, meaning that the executable part (dynamic library here) of the file bundle must match the name of the framework and be located at its root.

This is only enabled for Darwin platforms, filtered by the __APPLE__ define. On iOS and tvOS, this merge request also change the directory from which the plugins are loaded on iOS and tvOS since those platforms cannot have subdirectories for frameworks when shipping applications.